With the development of the future service, next generation wireless communication systems, such as 3GPP (third Generation Partnership Project) LTE (long term evolution) and beyond system, IMT-A (International Mobile Telecommunications-Advanced) system etc. are introduced to satisfy high speed, large capacity, and a high QoS (Quality of Service) for billions of subscribers. The D2D communication is introduced as a key feature to be supported by the next generation wireless communication systems. In a D2D communication, traffics are transported directly between two user equipments via a D2D communication path without using any network infrastructure (e.g. comprising the radio access network and the core network). In comparison with a cellular communication in which traffics are transported between two user equipments via an infrastructure communication path using the network infrastructure, a D2D communication can bring in the following advantages: power saving, capacity improvement, higher bit-rate and lower service cost etc. for end user perspective; offload of the core network, higher revenue, larger market penetration and new services for network operator.
As such, when radio link conditions for a D2D communication is favorable, for example if two user equipments are close to each other, and/or if a D2D communication actually offers higher throughput than a cellular communication, it would benefit the end users and the network operator by using a D2D communication path for transporting traffic directly between the two user equipments, instead of using an infrastructure communication path. Then, there exist a need to switch from an infrastructure communication path to a D2D communication path. Furthermore, when two equipments are in a D2D communication, transport conditions for the D2D communication may become unfavorable. For example, if one of two user equipments leaves, and/or if a cellular communication can achieve higher throughput than a D2D communication, there may exist a need to switch from the D2D communication path to an infrastructure communication path.
Generally, using a D2D communication path or an infrastructure communication path is transparent to the end user (i.e., the end user does not have to select a special option for using whether a D2D communication path or an infrastructure communication path). Thus, it is desired to not cause the service interruption when switch between a D2D communication path and an infrastructure communication path, to guarantee user satisfaction.
From this issue, it would be advancement in the art to provide solutions that allow for seamless switch between a D2D communication path and an infrastructure communication path.